decimals don't really give you something that's easily recognizable to factor out. what i would do here is try and divide 9.6 by 2.4 to see what's left if you factor out 2.4 as a whole.
9.6/2.4 = 4
so if 4 is what's left over there, factor out the 2.4 ... 2.4(n + 4) is your result.
when given decimals, i usually try to divide it out of the constant (aka the number without a variable).